The field of the invention is apparatus for treating polymer film materials by exposing the surface of the film to a high voltage gaseous discharge having corona characteristics.
The treatment of both conductive and non-conductive materials to enhance their adhesion with printing inks is a well-established process. A sheet, or web, of a polymer material to be treated, for example, is conveyed over a metal roller which forms one electrode of the treatment apparatus. The other electrode is spaced from the metal roller and a high voltage alternating current is applied across the electrodes. The corona discharge in the air gap between the electrode bombards the surface of the polymer material with electrons and ions to physically and chemically change its surface as it moves over the metal roller. The amount of treatment may be varied and is dependent on the degree of the adhesion problem presented by the selected polymers and inks. Apparatus for carrying out this treatment process is described in U.S. Pat. Nos. 3,133,193; 3,507,763; 3,662,169; 3,708,733; 3,817,701; 3,900,538; 3,973,132 and 4,051,044.
When the material being treated is relatively thin, special precautions must be taken to prevent arc-over from occurring during the treatment process. Such arching burns holes through the treated material and draws excessive currents which trip the power supply circuit breakers. Although arc-over can be prevented by reducing the applied voltage, this also reduces the degree of surface treatment and is unsatisfactory in most instances. Instead, the solution to this problem has been to coat the metal roller with a layer of insulating material having a high dielectric constant. Although this is an effective solution which has been employed for more than a decade, the dielectric layer adds considerable expense and requires periodic replacement. There is a need for an apparatus which effectively treats thin polymer materials without the need for a dielectric layer on the metal roller.